The present invention relates to a pneumatic tire having a tread portion made of a low-hysteresis-loss rubber compound and static electricity grounding means.
In recent years, in order to lessen the fuel consumption of vehicles, pneumatic tires having a tread portion made of a rubber compound mainly reinforced by silica instead of carbon black have been proposed. The tread rubber therefore, shows a low hysteresis loss, and as a result, the rolling resistance of the tire can be decreased. In such a silica-reinforced rubber, however, the electrical resistance is very high. Therefore, it is necessary to provide a means of eliminating static electricity of vehicles.
In the laid-open Japanese patent application No. JP-A-9-71112, for example, a pneumatic tire having static electricity grounding means is disclosed, wherein the tread portion (A) is made of a silica-reinforced rubber (b) and an electrically conductive carbon-reinforced rubber (a) as shown in FIG. 4, and the conductive carbon-reinforced rubber (a) is provided with parts (a1) extending radially outwardly to the ground contacting face (bs) of the tread portion.
In this structure, the ground contacting region is defined by two kinds of rubber compounds. Therefore, uneven wear is liable to occur therebetween. If the radially outwardly extending parts (a1) wear faster than the tread rubber (b), the electrical resistance between the tire and the ground increases. If the tread rubber (b) wears faster than the parts (a1), there is a possibility that the grip performance deteriorates. Further, it is difficult to form the radially extending parts (a1) with accuracy because the rubber is softened during vulcanizing the tire and flows more or less. Furthermore, an increase in the tire making processes and/or an increase in the facility cost may be unavoidable.
It is therefore, an object of the present invention to provide a pneumatic tire which can solve the above-mentioned problems, namely, the increase in the electrical resistance and deterioration of the grip performance due to the uneven wear, and the lowering of accuracy, and the increase in the tire making processes and/or facility cost.
According to the present invention, a pneumatic tire comprises
a tread rubber extending over a ground contacting region of the tire,
tread grooves disposed in the ground contacting region, static electricity grounding means comprising grounding terminals and a collector,
the collector and grounding terminals being made of a conductive rubber compound containing at least 15 phr of carbon black to have a volume resistivity of less than 100 megaohm cm, and
the tread rubber being made of a low-hysteresis-loss rubber compound containing silica as a reinforcing agent and having a volume resistivity more than that of the electrically conductive rubber compound, wherein
the collector is disposed radially inside the tread grooves, and the grounding terminals are disposed in the tread grooves and extend radially outwardly from the collector to the level of the ground contacting region at least.